Hi Vaeshtka Yvormes
GTC's purchased directly from other players on the forum, using ISK, has to be traded through secure sale option in account management.
When seller sets this up, the buyer has 48 hours to accept the offer.
When offer is accepted the gametime is added directly to the buyers account. This gametime cannot
be exchanged for PLEX
The rules for GTC trade can be found here
If you buy your 60-day GTC directly from an authorized seller for real money, you can use the ingame "Convert ETC" option to turn it into 2 PLEX's.
If what you are looking for are PLEXs, I would suggest you use the ingame market rather than the Timecode Bazaar on the forum.